Prisoner, maybe, one of an exclusive group (7)
I believe the answer is:
insider
This clue is a double definition.
'prisoner maybe' is the first definition.
(someone who's 'inside')
'one of an exclusive group' is the second definition.
(someone inside a particular group)
(Other definitions for insider that I've seen before include "Accepted member of accepted group, has access to information" , "Privileged member of a group" , "I may know secrets" , "Member of clique" , "Particularly advantaged person" .)